Impact of total organic carbon (in sediments) and dissolved organic carbon (in overlying water column) on Hg sequestration by coastal sediments from the central east coast of India

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  • Marine Pollution Bulletin 79 (2014) 342347

    Contents lists available at ScienceDirect

    Marine Pollution Bulletin

    journal homepage: www.elsevier .com/locate /marpolbul

    Baseline

    Impact of total organic carbon (in sediments) and dissolved organiccarbon (in overlying water column) on Hg sequestration by coastalsediments from the central east coast of India

    0025-326X/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.marpolbul.2013.11.028

    Corresponding author. Tel.: +91 8322450 495; fax: +91 8322450 602.E-mail address: pchak@nio.org (P. Chakraborty).

    Parthasarathi Chakraborty a,, Brijmohan Sharma a,b, P.V. Raghunath Babu a, Koffi Marcellin Yao a,c,Saranya Jaychandran a,d

    a Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa 403004, Indiab TERI University, 10, Institutional Area, Vasant Kunj, New Delhi, Indiac Centre de Recherches Oceanologiques, BP V18 Abidjan, Cote dIvoired Cochin University of Science and Technology, Cochin 682022, Kerala, India

    a r t i c l e i n f o a b s t r a c t

    Keywords:Hg-sediment interactionHg speciationHgTOC complexesHumic acidHg complexes

    Total organic carbon (TOC) (in sediment) and dissolved organic matter (DOM) (in water column) playimportant roles in controlling the mercury sequestration process by the sediments from the central eastcoast of India. This toxic metal prefers to associate with finer size particles (silt and clay) of sediments.Increasing concentrations of DOM in overlying water column may increase complexation/reduction pro-cesses of Hg2+ within the water column and decrease the process of Hg sequestration by sediments. How-ever, high concentrations of DOM in water column may increase Hg sequestration process by sediments.

    2013 Elsevier Ltd. All rights reserved.

    Mercury (Hg) has received a worldwide attention due to its sig-nificant global adverse impact on both environment and humanhealth (Ratcliffe et al., 1996; Boening, 2000; Wolfe et al., 2009).Due to its high toxicity, distributions of Hg compounds in coastal,estuarine and marine environments have been the subject of manyresearchers (Valette-Silver, 1993; Daskalakis and OConnor,1995;Long et al., 1995; Gagnon et al., 1997; Kannan and Falandysz,1998; Kannan et al., 1998; Horvat et al., 1999; Borja et al., 2000;Hines et al., 2000). It is well known that sediment, an integraland inseparable part of aquatic system, plays an important rolein controlling Hg pollution in aquatic systems (Farmer, 1991; Tackand Verloo, 1995; Bubb and Lester, 1996; Liu et al., 2006). Hg accu-mulates in sediments globally from many physical, chemical, bio-logical, geological and anthropogenic environmental processes.Thus, sediment can be a good indicator of water quality of a partic-ular area (Dai et al., 2007; Yu et al., 2008; Bilotta and Brazier,2008).

    The toxicity and bioavailability of Hg is dependent on its chem-ical speciation rather than its total concentrations in sediments. Insediments, Hg can be associated with different binding phases.These forms, or species, are of key importance in risk assessmentsof Hg in sediments (as well as in the overlying water column).However, determination of Hg associated with different bindingphases in sediments is a very difficult task due to intrinsic

    complexity of binding sites in sediment (Chakraborty, 2010,2012, 2012a, 2013).

    In addition to that there are several factors (dissolved organicmatter in water column, adsorption capacity of sediments,different type of binding sites, pH of the water column, chlorideion concentrations, ionic strength, cation exchange capacity, oxida-tionreduction potential) may also influence the distribution andspeciation of metals in sediments (Chakraborty and Chakrabarti,2006, 2008; Gopalapillai et al., 2008; Chakraborty, 2010; Chakr-aborty et al., 2010, 2012b,c). Sequential extraction procedures(SEP) are designed to determine metal contents associated to dif-ferent solid phases present in sediments. These methods lead toobtain operationally defined fractions related to the mobility andpotential toxicity of metals and rarely with specific individualspecies. Several operationally-defined sequential extraction proce-dures have been widely used to assess the bioavailable Hg fractionand their mobility in sediment. In this work a modified BCR(sequential extraction) method was used for the evaluation ofthe geochemistry of Hg and determines non-residual (bioavailable)Hg species and phases in the coastal sediment from the central eastcoast of India. The aims of this study were also to identify thefactors which control Hg-sediment interaction and elucidate theimpacts of DOM (in this case humic acid in the overlying watercolumn) on the sequestration processes of Hg by the coastal sedi-ments from the central east coast of India.

    Sediment samples were collected from the four different envi-ronmentally significant sites of the central east coast of India as

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  • Fig. 2. The schematic diagram of the modified BCR protocol for Hg fractionationstudy in coastal sediments.

    Table 1Percentages of silt, clay, sand, TOC, total Hg, in the coastal sediments.

    Station Sand(%)

    Silt(%)

    Clay(%)

    TOC(%)

    [Hg]T (lg/kg)

    Bhimili 67.2 15.8 20.4 0.06 16.6Visakhapatnam 58.2 39.3 2.5 0.12 28.9Gangavaram 14.2 50.4 35.4 0.42 59.4Kakinada 6.6 88 5.4 0.24 88.4

    P. Chakraborty et al. / Marine Pollution Bulletin 79 (2014) 342347 343

    shown in the Fig. 1. The coastal sediments were collected from (1)Bhimili, (2) Visakhapatnam, (3) Gangavaram, and (4) Kakinada. Thegeneral description, geographic location of the sampling sites, thedistance from the shore, and the depth from where the sedimentsamples were collected are given as supporting document(Table 1SD).

    A Van Veen stainless steel grab sampler with an area of 0.02 m2

    was used to collect the sediments; without emptying the grab, asample was taken from the centre with a polyethylene spoon toavoid contamination by metallic parts of the dredge. The sampleswere stored at 20 C for 15 days, and then dried at room temper-ature (2530 C) by keeping the sediment samples on Petri dishes(covered with parafilm). However, the sediment samples (of largequantity) were also dried at 60 C in a forced air oven (Kadavil Elec-tro Mechanical Industry Pvt. Ltd. India, Model No. KOMS.6FD).There was no loss of Hg found from the oven dried sediment com-pared to the sediments dried at room temperature.

    A series of batch extractions were performed on the coastal sed-iments, following BCR protocol (Quevauviller et al., 1993; Ure et al.,1993). This sequential extraction protocol allows us to determinethe sum of ion-exchangeable, water soluble and carbonate/bicar-bonate form of Hg (Fr. 1): Hg bound to FeMn oxides i.e., reduciblefraction of Hg (Fr. 2); fraction of total Hg bound to organic matter(Fr. 3) and residual Hg fraction (Fr. 4) in sediment. The protocol ispresented in Fig. 2.

    Total Hg was analyzed by direct mercury Analyzer (Tri cellDMA-80) from Milestone, Italy. Operational conditions were estab-lished according to EPA method 7473 (EPA, 2007). The sensitivityand the accuracy of the DMA was monitored in each experimentby analyzing certified reference material (Reference marine sedi-ments (MESS-2, PACS-1) from Natural Resources of Canada)followed by a reagent blank between every 10 samples. total or-ganic carbon (TOC) in the studied sediments was determined byfollowing the WalkleyBlack method (Schumacher, 2002).

    The general description and texture analysis of the studied sed-iments are presented in Table 1. The total Hg content in the coastalsediments was found to vary from 16.6 to 88.4 lg kg1. The max-imum concentration of Hg was found in the sediment collectedat Kakinada (88.4 lg kg1) followed by Gangavaram (59.4 lg kg1),Visakhapatnam (28. 9 lg kg1) and Bhimli (16. 6 lg kg1). It isinteresting to note that the total Hg content in the coastal sedi-ments was found to increase from north to south of the central east

    Fig. 1. Different environmentally significant sam

    coast of India. The concentrations of the finer size (sum of silt andclay contents) particles was found to influence the accumulation ofHg in the studied sediments. A positive correlation (R2 = 0.96) wasfound between the total Hg content and the finer particles

    pling sites off the central east coast of India.

  • Fig. 3. Distribution of Hg in different binding phases of the coastal sediments (byfollowing BCR protocol).

    344 P. Chakraborty et al. / Marine Pollution Bulletin 79 (2014) 342347

    fractions in the coastal sediments. Yoshida et al. (2006) have alsoreported that finer particles in sediments strongly influence thedistribution of Hg between sediment and the overlying watercolumn.

    TOC content in the studied sediment was found to increase withthe increasing concentrations of the finer particles and decreasewith the increasing coarser fraction (sand) in the studied sedi-ments. A strong correlation (R2 = 0.83) was found between theTOC concentrations with the fraction of finer particles in the coast-al sediments. Percival et al. (2000) has reported that finer particlesof sediment control organic carbon level in sediments. A strongcorrelation (R2 = 0.92) between the total concentrations of Hg withthe TOC content in the sediments (as shown in Table 2) indicatesthat TOC in the coastal sediments probably plays an important rolein controlling Hg sequestration process by the sediment from thecentral east coast of India.

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